x86, fpu: decouple non-lazy/eager fpu restore from xsave

Decouple non-lazy/eager fpu restore policy from the existence of the xsave
feature. Introduce a synthetic CPUID flag to represent the eagerfpu
policy. "eagerfpu=on" boot paramter will enable the policy.

Requested-by: H. Peter Anvin <hpa@zytor.com>
Requested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1347300665-6209-2-git-send-email-suresh.b.siddha@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Documentation/kernel-parameters.txt

@@ -1833,6 +1833,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			and restore using xsave. The kernel will fallback to
 			enabling legacy floating-point and sse state.
 
+	eagerfpu=	[X86]
+			on	enable eager fpu restore
+			off	disable eager fpu restore
+
 	nohlt		[BUGS=ARM,SH] Tells the kernel that the sleep(SH) or
 			wfi(ARM) instruction doesn't work correctly and not to
 			use it. This is also useful when using JTAG debugger.

arch/x86/include/asm/cpufeature.h

@@ -97,6 +97,7 @@
 #define X86_FEATURE_EXTD_APICID	(3*32+26) /* has extended APICID (8 bits) */
 #define X86_FEATURE_AMD_DCM     (3*32+27) /* multi-node processor */
 #define X86_FEATURE_APERFMPERF	(3*32+28) /* APERFMPERF */
+#define X86_FEATURE_EAGER_FPU	(3*32+29) /* "eagerfpu" Non lazy FPU restore */
 
 /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
 #define X86_FEATURE_XMM3	(4*32+ 0) /* "pni" SSE-3 */
@@ -305,6 +306,7 @@ extern const char * const x86_power_flags[32];
 #define cpu_has_perfctr_core	boot_cpu_has(X86_FEATURE_PERFCTR_CORE)
 #define cpu_has_cx8		boot_cpu_has(X86_FEATURE_CX8)
 #define cpu_has_cx16		boot_cpu_has(X86_FEATURE_CX16)
+#define cpu_has_eager_fpu	boot_cpu_has(X86_FEATURE_EAGER_FPU)
 
 #if defined(CONFIG_X86_INVLPG) || defined(CONFIG_X86_64)
 # define cpu_has_invlpg		1

arch/x86/include/asm/fpu-internal.h

@@ -38,6 +38,7 @@ int ia32_setup_frame(int sig, struct k_sigaction *ka,
 
 extern unsigned int mxcsr_feature_mask;
 extern void fpu_init(void);
+extern void eager_fpu_init(void);
 
 DECLARE_PER_CPU(struct task_struct *, fpu_owner_task);
 
@@ -84,6 +85,11 @@ static inline int is_x32_frame(void)
 
 #define X87_FSW_ES (1 << 7)	/* Exception Summary */
 
+static __always_inline __pure bool use_eager_fpu(void)
+{
+	return static_cpu_has(X86_FEATURE_EAGER_FPU);
+}
+
 static __always_inline __pure bool use_xsaveopt(void)
 {
 	return static_cpu_has(X86_FEATURE_XSAVEOPT);
@@ -99,6 +105,14 @@ static __always_inline __pure bool use_fxsr(void)
         return static_cpu_has(X86_FEATURE_FXSR);
 }
 
+static inline void fx_finit(struct i387_fxsave_struct *fx)
+{
+	memset(fx, 0, xstate_size);
+	fx->cwd = 0x37f;
+	if (cpu_has_xmm)
+		fx->mxcsr = MXCSR_DEFAULT;
+}
+
 extern void __sanitize_i387_state(struct task_struct *);
 
 static inline void sanitize_i387_state(struct task_struct *tsk)
@@ -291,13 +305,13 @@ static inline void __thread_set_has_fpu(struct task_struct *tsk)
 static inline void __thread_fpu_end(struct task_struct *tsk)
 {
 	__thread_clear_has_fpu(tsk);
-	if (!use_xsave())
+	if (!use_eager_fpu())
 		stts();
 }
 
 static inline void __thread_fpu_begin(struct task_struct *tsk)
 {
-	if (!use_xsave())
+	if (!use_eager_fpu())
 		clts();
 	__thread_set_has_fpu(tsk);
 }
@@ -327,10 +341,14 @@ static inline void drop_fpu(struct task_struct *tsk)
 
 static inline void drop_init_fpu(struct task_struct *tsk)
 {
-	if (!use_xsave())
+	if (!use_eager_fpu())
 		drop_fpu(tsk);
-	else
-		xrstor_state(init_xstate_buf, -1);
+	else {
+		if (use_xsave())
+			xrstor_state(init_xstate_buf, -1);
+		else
+			fxrstor_checking(&init_xstate_buf->i387);
+	}
 }
 
 /*
@@ -370,7 +388,7 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
 	 * If the task has used the math, pre-load the FPU on xsave processors
 	 * or if the past 5 consecutive context-switches used math.
 	 */
-	fpu.preload = tsk_used_math(new) && (use_xsave() ||
+	fpu.preload = tsk_used_math(new) && (use_eager_fpu() ||
 					     new->fpu_counter > 5);
 	if (__thread_has_fpu(old)) {
 		if (!__save_init_fpu(old))
@@ -383,14 +401,14 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
 			new->fpu_counter++;
 			__thread_set_has_fpu(new);
 			prefetch(new->thread.fpu.state);
-		} else if (!use_xsave())
+		} else if (!use_eager_fpu())
 			stts();
 	} else {
 		old->fpu_counter = 0;
 		old->thread.fpu.last_cpu = ~0;
 		if (fpu.preload) {
 			new->fpu_counter++;
-			if (!use_xsave() && fpu_lazy_restore(new, cpu))
+			if (!use_eager_fpu() && fpu_lazy_restore(new, cpu))
 				fpu.preload = 0;
 			else
 				prefetch(new->thread.fpu.state);
@@ -452,15 +470,23 @@ static inline void user_fpu_begin(void)
 	preempt_enable();
 }
 
+static inline void __save_fpu(struct task_struct *tsk)
+{
+	if (use_xsave())
+		xsave_state(&tsk->thread.fpu.state->xsave, -1);
+	else
+		fpu_fxsave(&tsk->thread.fpu);
+}
+
 /*
  * These disable preemption on their own and are safe
  */
 static inline void save_init_fpu(struct task_struct *tsk)
 {
 	WARN_ON_ONCE(!__thread_has_fpu(tsk));
 
-	if (use_xsave()) {
-		xsave_state(&tsk->thread.fpu.state->xsave, -1);
+	if (use_eager_fpu()) {
+		__save_fpu(tsk);
 		return;
 	}
 
@@ -526,11 +552,9 @@ static inline void fpu_free(struct fpu *fpu)
 
 static inline void fpu_copy(struct task_struct *dst, struct task_struct *src)
 {
-	if (use_xsave()) {
-		struct xsave_struct *xsave = &dst->thread.fpu.state->xsave;
-
-		memset(&xsave->xsave_hdr, 0, sizeof(struct xsave_hdr_struct));
-		xsave_state(xsave, -1);
+	if (use_eager_fpu()) {
+		memset(&dst->thread.fpu.state->xsave, 0, xstate_size);
+		__save_fpu(dst);
 	} else {
 		struct fpu *dfpu = &dst->thread.fpu;
 		struct fpu *sfpu = &src->thread.fpu;

arch/x86/kernel/cpu/common.c

@@ -1297,7 +1297,6 @@ void __cpuinit cpu_init(void)
 	dbg_restore_debug_regs();
 
 	fpu_init();
-	xsave_init();
 
 	raw_local_save_flags(kernel_eflags);
 
@@ -1352,6 +1351,5 @@ void __cpuinit cpu_init(void)
 	dbg_restore_debug_regs();
 
 	fpu_init();
-	xsave_init();
 }
 #endif

arch/x86/kernel/i387.c

@@ -22,9 +22,8 @@
 /*
  * Were we in an interrupt that interrupted kernel mode?
  *
- * For now, on xsave platforms we will return interrupted
- * kernel FPU as not-idle. TBD: As we use non-lazy FPU restore
- * for xsave platforms, ideally we can change the return value
+ * For now, with eagerfpu we will return interrupted kernel FPU
+ * state as not-idle. TBD: Ideally we can change the return value
  * to something like __thread_has_fpu(current). But we need to
  * be careful of doing __thread_clear_has_fpu() before saving
  * the FPU etc for supporting nested uses etc. For now, take
@@ -38,7 +37,7 @@
  */
 static inline bool interrupted_kernel_fpu_idle(void)
 {
-	if (use_xsave())
+	if (use_eager_fpu())
 		return 0;
 
 	return !__thread_has_fpu(current) &&
@@ -84,7 +83,7 @@ void kernel_fpu_begin(void)
 		__save_init_fpu(me);
 		__thread_clear_has_fpu(me);
 		/* We do 'stts()' in kernel_fpu_end() */
-	} else if (!use_xsave()) {
+	} else if (!use_eager_fpu()) {
 		this_cpu_write(fpu_owner_task, NULL);
 		clts();
 	}
@@ -93,7 +92,7 @@ EXPORT_SYMBOL(kernel_fpu_begin);
 
 void kernel_fpu_end(void)
 {
-	if (use_xsave())
+	if (use_eager_fpu())
 		math_state_restore();
 	else
 		stts();
@@ -122,7 +121,6 @@ static void __cpuinit mxcsr_feature_mask_init(void)
 {
 	unsigned long mask = 0;
 
-	clts();
 	if (cpu_has_fxsr) {
 		memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct));
 		asm volatile("fxsave %0" : : "m" (fx_scratch));
@@ -131,7 +129,6 @@ static void __cpuinit mxcsr_feature_mask_init(void)
 			mask = 0x0000ffbf;
 	}
 	mxcsr_feature_mask &= mask;
-	stts();
 }
 
 static void __cpuinit init_thread_xstate(void)
@@ -185,9 +182,8 @@ void __cpuinit fpu_init(void)
 		init_thread_xstate();
 
 	mxcsr_feature_mask_init();
-	/* clean state in init */
-	current_thread_info()->status = 0;
-	clear_used_math();
+	xsave_init();
+	eager_fpu_init();
 }
 
 void fpu_finit(struct fpu *fpu)
@@ -198,12 +194,7 @@ void fpu_finit(struct fpu *fpu)
 	}
 
 	if (cpu_has_fxsr) {
-		struct i387_fxsave_struct *fx = &fpu->state->fxsave;
-
-		memset(fx, 0, xstate_size);
-		fx->cwd = 0x37f;
-		if (cpu_has_xmm)
-			fx->mxcsr = MXCSR_DEFAULT;
+		fx_finit(&fpu->state->fxsave);
 	} else {
 		struct i387_fsave_struct *fp = &fpu->state->fsave;
 		memset(fp, 0, xstate_size);

arch/x86/kernel/process.c

@@ -156,7 +156,7 @@ void flush_thread(void)
 	 * Free the FPU state for non xsave platforms. They get reallocated
 	 * lazily at the first use.
 	 */
-	if (!use_xsave())
+	if (!use_eager_fpu())
 		free_thread_xstate(tsk);
 }
 

arch/x86/kernel/traps.c

@@ -630,7 +630,7 @@ EXPORT_SYMBOL_GPL(math_state_restore);
 dotraplinkage void __kprobes
 do_device_not_available(struct pt_regs *regs, long error_code)
 {
-	BUG_ON(use_xsave());
+	BUG_ON(use_eager_fpu());
 
 #ifdef CONFIG_MATH_EMULATION
 	if (read_cr0() & X86_CR0_EM) {